Double Crucible Czochralski Research Articles

Crystal growth of ultra-large MgF2 and LiCaAlF6 single crystals by a double-crucible Czochralski technique

We report the successful growth of ultra-large sized magnesium fluoride (MgF2) and lithium calcium aluminum fluoride (LiCaAlF6) single crystals using the Czochralski growth method with a double-layer crucible. The 8-inch diameter MgF2 crystal was grown in argon atmosphere at a growth rate of 2 mm/h up to a length of 10 cm while the 4-inch diameter LiCaAlF6 crystal was grown in carbon tetrafluoride atmosphere at a growth rate of 1 mm/h up to a length of 30 cm. The 8-inch diameter satisfies the required size for the moderator that slows down neutrons in Boron Neutron Capture Therapy (BNCT), a noninvasive radiation therapy that selectively targets only the cancer cells. The successful growth of these ultra-large single crystals will not only advance BNCT but will also open new possibilities for optical materials such as lenses and windows in the vacuum ultraviolet region.

Formation of Self-Assembled Domain Structures in MgOSLT

The formation of self-organized structures of submicron domains under application of the uniform electric field was studied in single crystals of magnesium doped near stoichiometric lithium tantalate grown by double crucible Czochralski method. It was shown that the period of the domain structure (about 1 μm) appeared on Z− polar surface after polarization reversal was almost independent on the electric field. This fact confirms the self-organized nature of the phenomenon. The obtained dependence of the structure geometry on the switching conditions was discussed. The noticeable increase of the low-frequency dielectric permittivity was demonstrated in the samples with charged domain walls.

Crystal growth of Bi2TeO5 by a double crucible Czochralski method

Single crystals of bismuth tellurite (Bi2TeO5) were grown by a double crucible Czochralski method at different pulling and rotation rates, ranging from 0.1mm/h to 0.8mm/h and from 5rpm to 20rpm, respectively. The crystallized phase was verified by X-ray diffraction measurements, occurring structural defects were characterized, and the growth conditions were studied being determined the better parameters to growth good-quality single crystals with uniform pale yellow coloration. Optical band gap of 3.1eV was determined from the optical transmission spectrum.

Growth of KTN crystals by double crucible Czochralski method

Potassium tantalite niobate (KTa1−xNbxO3, KTN) single crystals with various compositions and sizes were grown by double crucible Czochralski method in KTaO3–KNbO3 solid solution system. All the crystals we have grown are in good transparence and are >1 cm in size. Growth habit, morphological, structural and homogeneity properties of five different compositions of KTN crystals, including KTaO3, KTa0·91Nb0·09O3, KTa0·81Nb0·19O3, KTa0·75Nb0·25O3 and KTa0·63Nb0·37O3, are investigated in this work. Electron probe microanalysis shows that the composition fluctuation of KTN crystal is <0·0003 cm−1. X-ray diffraction experimental results show that all crystals belong to cubic phase at room temperature. Lattice parameters a are 3·984∼3·994 Å, which corresponded to the composition of x = 0∼0·37. High resolution XRD measurement shows that all KTN crystals are in high integrity and good quality. Our work shows that the KTN crystals we have grown are in good quality and have a broad application prospects.

Czochralski growth of Ga 1−xIn xSb single crystals with uniform compositions

Severe macrosegregation tends to develop along Ga 1− x In x Sb alloy crystals grown by the conventional Czochralski process. Failure to eliminate macrosegregation by floating-crucible Czochralski has been attributed to InSb back diffusion from the floating crucible. In the present study Ga 1− x In x Sb crystals were grown from the growth melt in a floating crucible having a unique bottom tube that was wide to let the replenishing melt in the outer crucible pass through easily but long to suppress back diffusion. Ga 1− x In x Sb crystals with a uniform composition of 1 mol% InSb were grown as targeted. However, crystals of about 2 mol% InSb were grown when 4 mol% InSb was targeted. By using Bi 1− x Sb x as a model material, it was found that hydrodynamic instability caused the problem—the denser growth melt mixed with the lighter replenishing melt during or even before crystal growth. A new double-crucible Czochralski process was thus developed, with an upper chamber for the growth melt and a lower chamber for the replenishing melt. The slightly higher gas pressure in the lower chamber caused replenishing through a long capillary tube that suppressed mixing between the melts. Single crystals of Ga 1− x In x Sb were grown with uniform compositions up to 4.5 mol% InSb.

Growth of 4-in diameter near-stoichiometric lithium tantalate single crystals

We present the aspects on the growth of 4-in diameter near-stoichiometric lithium tantalate (SLT) crystals, from lithium-enriched melt, employing double crucible Czochralski (DCCZ) technique. The difficulties encountered during growth such as crack and mechanical twin formation are discussed in detail and the same overcame by adopting suitable experimental parameters. The wafers sliced from the crystals are found to be compositionally homogeneous and free from sub-grains. The variation in the refractive indices across the wafer is of the order of 10 −5. We could not observe any marked difference either in the absorption edge or poling field for domain inversion at different locations in the wafer.

Electron paramagnetic resonance of Cr3+ in near-stoichiometric LiTaO3

Electron-paramagnetic-resonance (EPR) experiments on the dominant Cr3+ center in near-stoichiometric LiTaO3 crystals, grown by the double crucible Czochralski method, are reported. A near complete roadmap of EPR positions was obtained allowing an accurate determination of the spin-Hamiltonian parameters. Newman superposition model calculations of zero-field splitting term were performed and support the model of Cr3+ incorporation within the Li octahedron. Calculations were also made for Cr3+ in LiNbO3, again good agreement with a Li site model was obtained for the main EPR center. The temperature dependence of the zero-field splitting parameter for Cr3+ in LiTaO3 was found to show anomalous behavior in the region of 40 K, suggesting the presence local structural instability at the ion site.

Electron paramagnetic resonance ofFe3+ innear-stoichiometric LiTaO3

Electron paramagnetic resonance (EPR) experiments on the dominantFe3+ centre innear-stoichiometric LiTaO3 crystals grown by the double crucible Czochralski method are reported. Anear complete roadmap of EPR positions was obtained, and transitionsfrom two magnetically non-equivalent sites clearly resolved in thezx plane, perpendicular to the glide plane. This allowed accurate determination ofC3 symmetry spin Hamiltonian parameters. Newman superposition model analyses of secondand fourth order zero field splitting term parameters were performed to give further insightinto the site of incorporation. The second order calculations provide evidence forFe3+ substitution within the Li octahedron.

Thermal stress inhibition in double crucible Czochralski large diameter crystal growth

In this paper we consider the problem of thermal stress inhibition in double crucible Czochralski crystal growth by means of a thermal tube with linear temperature distribution computationally determined from solution with adiabatic boundary condition, which provides the formation of a stressless one-dimensional temperature distribution along the growing crystal. We study how this linear temperature distribution applied along the thermal tube is “transferred” into the growing crystal depending on the gap between crystal and inner tube surfaces, within which complex radiative–convective heat transfer takes place. Calculations presented in the paper suggest that linear temperature distribution along the thermal tube allows an order of magnitude stress reduction in 80–100 mm diameter crystals of LiNbO 3

Complex dispersion relation of phonon‐polariton in stoichiometric LiNbO 3

Complex polariton dispersion relations were studied in the nearly stoichiometric lithium niobate crystals grown by the double crucible Czochralski method using the terahertz (THz) time domain spectroscopy. The intensity and phase delay of transmitted THz radiation have been accurately measured for the propagation along the x-axis from 0.1 to 2.3 THz. The dispersion relation of phonon-polariton frequency and damping factor have been determined for A1(z) and E(x,y) symmetry polaritons. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of shape of an inner crucible on convection of lithium niobate melt in a double-crucible Czochralski process using the accelerated crucible rotation technique

Numerical analysis was carried out to clarify melt convection of LiNbO 3 in a double-crucible Czochralski process using the accelerated crucible rotation technique. We investigated the effects of window shape of an inner crucible on melt convection in a double crucible. It was found that the height of windows of an inner crucible has a large effect in comparison with the effect of width of windows on melt convection. When the window height was set high, melt in the outer crucible could easily flow into the inner crucible. Then it effectively modified heat transfer near the melt–crystal interface. On the other hand, when the window height was set low, cell formation was observed near the melt–crystal interface.

Effects of shape of an inner crucible on convection of lithium niobate melt in a double-crucible Czochralski process using the accelerated crucible rotation technique

Numerical analysis was carried out to clarify melt convection of LiNbO 3 in a double-crucible Czochralski process using the accelerated crucible rotation technique. We investigated the effects of window shape of an inner crucible on melt convection in a double crucible. It was found that the height of windows of an inner crucible has a large effect in comparison with the effect of width of windows on melt convection. When the window height was set high, melt in the outer crucible could easily flow into the inner crucible. Then it effectively modified heat transfer near the melt–crystal interface. On the other hand, when the window height was set low, cell formation was observed near the melt–crystal interface.

Domain structures and etching morphologies of lithium niobate crystals with different Li contents grown by TSSG and double crucible Czochralski method

AbstractStoichiometric lithium niobate single crystals with different Li contents have been grown both by the top‐seeded solution growth (TSSG) method from potassium containing flux and by the double crucible Czochralski (DC Cz) method. Spectroscopic properties (e.g. the UV absorption edge, Raman linewidth) and the Curie temperature measurement have been used for the characterization of the crystal composition. The double crucible Czochralski method is found to be suitable for mass production of stoichiometric LiNbO3 with Li content larger than 49.7 mol% and homogeneity of 0.03 mol%. The domain structures and etching morphologies on negative and positive c‐surface were also investigated by chemical etching method. A new domain structure of three‐fold symmetric sectors were observed in near‐stoichiometric LiNbO3 grown by TSSG method. The straight line arrangement hillocks on negative c‐surface and the net‐like arrangement etch lines were observed and explained by stress etching mechanism. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Numerical analysis of continuous charge of lithium niobate in a double-crucible Czochralski system using the accelerated crucible rotation technique

The transport mechanism of supplied raw material in a double-crucible Czochralski system using the accelerated crucible rotation technique (ACRT) was investigated by three-dimensional and time-dependent numerical simulation. The calculation clarified that use of the ACRT resulted in enhancement of the mixing effect of the supplied raw material. It is, therefore, possible to maintain the composition of the melt in an inner crucible during crystal growth by using the ACRT. The effect of the continuous charge of the raw material on melt temperature was also investigated. Our results showed that the effect of feeding lithium niobate granules on melt temperature was small, since the feeding rate of the granules is small. Therefore, solidification of the melt surface due to the heat of fusion in this system is not likely.

Influence of powder supply on radio-frequency power stability and compositional uniformity in near-stoichiometric LiTaO 3 crystal grown by double-crucible Czochralski method

Near-stoichiometric LiTaO 3 (SLT) crystals were grown from a Li-rich solution by the double-crucible Czochralski (DCCZ) method with and without an automatic powder supply system, to investigate influence of the powder supply system on the stability of radio-frequency (RF) power and the compositional uniformity of the grown crystal. To maintain a constant crystal diameter, the RF power is decreased during crystal growth without the powder supply system, while it is kept almost constant during crystal growth with the powder supply system. The Curie temperature of an SLT crystal grown with the powder supply system was almost constant along both growth and lateral directions, and the [Li]/([Li]+[Ta]) ratio in this crystal was estimated to be 0.4992 by comparing its Curie temperature with those of ceramics of known composition. In contrast, in an SLT crystal grown without the powder supply system, the Curie temperature was observed to increase with increase in distance from the top of the crystal; therefore, the crystal composition was thought to gradually change due to the change of solution composition during crystal growth. In SLT crystal growth, the DCCZ method using the automatic powder supply system provided an almost constant thermal condition during crystal growth as well as compositional uniformity throughout the SLT crystal.

Growth of Li xTa 1− xO 3 single crystals and their optical properties

Lithium tantalate (LiTaO 3) single crystals having different Li/Ta ratios were grown by using conventional and double crucible Czochralski method. The grown crystals appeared to have mole percent of Li 2O ranging from 45.5 to 50 mol%. We analyzed the real composition with ICP method and measured the Curie temperatures ( T c) with DTA method. The relationship between T c and the mole percent of Li 2O in LiTaO 3 crystals was derived. The transmission spectra and refractive indices were measured. The refractive indices measured by prism coupler were analyzed in terms of single-term Sellmeir parameters.

Improved Properties of Stoichiometric LiNbO3 for Electro-Optic Applications

High quality stoichiometric LiNbO3 single crystals, 40 mm in diameter and 80-100 mm in length, have been successfully grown by the double crucible Czochralski method equipped with automatic powder supply system using radio-frequency heating and automatic diameter control system. Although some difficulties still exist in growing 3 inch size single crystals from offcongruent composition melts, our results demonstrate that stoichiometric LiNbO3 crystals containing much less nonstoichiometric defects than congruent LiNbO3 exhibit some advantageous properties for holographic data storage applications. This was confirmed by the results of two-beam coupling experiment and digital hologram test. In digital holographic data storage, we have demonstrated that stoichiometric LiNbO3 shows more than 10 times larger photorefractive sensitivity than congruent LiNbO3 In order to interpret the high performance of stoichiometric LiNbO3, we compared several relevant parameters such as electro-optic constants, photoconductivity, and photogalvanic constants between stoichiometric and congruent LiNbO3 crystals. We report that material parameters are strongly dependent on the density of nonstoichiometric defects and control of these defects is of key importance in improving and tailoring those properties.

Stoichiometric LiTaO3 for Dynamic Holography in Near UV Wavelength Range

LiTaO3 single crystals with near-stoichiometric composition are grown using a double crucible Czochralski method equipped with an automatic powder supply system. Their photorefractive properties are compared with those of LiTaO3 crystals grown from the congruent composition melt. Two beam coupling experiments show that nearly stoichiometric LiTaO3 crystals exhibit considerably higher photorefractive gain, speed and sensitivity than conventional congruent crystals. Due to the relatively large photoconductivity observed at near UV wavelengths, stoichiometric crystals are further characterized by small photogalvanic fields, making them very attractive for dynamic holography in this wavelength range.

Stoichiometric LiTaO 3 single crystal growth by double crucible Czochralski method using automatic powder supply system

Stoichiometric LiTaO 3 single crystals, 45 mm in diameter and 50–85 mm in length, were grown by the double crucible Czochralski method equipped with automatic powder supply system using radio-frequency heating and automatic diameter control system. The crystals were grown from an off-congruent Li-rich melt (Li 2O 58–59 mol%) in the inner crucible made of Ir, and the stoichiometric powder was supplied smoothly and continuously to the outer melt at the rate of weight increase of the growing crystal as monitored by a load cell. The chemical composition of the crystal was confirmed by the Curie temperature measurements to be nearly stoichiometric (Li 2O 49.9 mol%).

Crystal growth and low coercive field 180° domain switching characteristics of stoichiometric LiTaO3

We grew LiTaO3 single crystals with a composition close to stoichiometry by using a double crucible Czochralski method. The switching field required for 180° ferroelectric domain reversal and the internal fields originating from nonstoichiometric point defects were compared for the stoichiometric and conventional commercially available crystals. The switching fields for the domain reversal in the stoichiometric crystal with a Curie temperature of 685 °C was 1.7 kV/mm. This is about one thirteenth of the switching field required for the conventional LiTaO3 crystals with a Curie temperature near 600 °C. The internal field in the stoichiometric crystal drastically decreased to 0.1 kV/mm.